This invention relates to a process, and apparatus for carrying out the process, for testing recording media, for example, magnetic tape of the type used for digital storage of data in the computer industry.
It is well known that magnetic tape is seldom free from all defects in magnetic storage capacity. This is true for newly-manufactured tape; but, it is particularly true of used magnetic tape where defects may have accrued during many writing and subsequent reading cycles. Scratches, presence of foreign debris such as dust, or presence of debris generated by the read and write equipment, can all contribute to the presence of an error. An error is any condition of the tape which results in the absence or non-functionality of the magnetic material which is supposed to form one or more information recording sites, i.e., one or more "bits". It has been recognized for some time that such tape should be tested for error content before reuse. Many customers require that the testing be carried out on new tape.
To understand the testing of magnetic tape, it is necessary to understand that a tape usually comrpises a number of parallel, data-containing tracks running along the length of the tape. The tape, on being used, will be magnetically encoded with information according to one of a number of industry-accepted formats. The formats presently in use include a so-called 6250 CPI, group code recorded (GRC) format and a 1600 BPI/3200 fci format. These formats provide that the tape be divided along its length into a pattern of a variety of segments each to carry information having a different function.
The most sophisticated of pre-existing tape testing apparatus is believed to operate as follows: The apparatus would detect a first error, assume it takes place at the start of a certain increment of the length of the tape, identify errors in the same increment, and assume that all errors in the same increment are cumulative. This procedure will report three one-track errors as one three-track error, two two-track errors and three one-track errors. In actual practice such numbers may be modified by appropriate steps. For example, the number of one-track errors is found by subtracting the number of two- and three-track errors from the total of errors to give the number of one-track errors and subtracting the number of three-track errors from the two-track errors to get the number of two-track errors. No correction is required for the number of three-track errors.
Thereupon, the old apparatus once again proceeds to test the tape and will take no further step to evaluate the tape until the next error causes it to evaluate the increment of the tape immediately following this next error. At this point, the apparatus will again scan the next increment of tape as indicated above. The magnetic tape industry has found this procedure of some use in giving what is at least a rough and comparative method for determining the error content in tape.
However, it is to be noted that there is a strong probability that the above-described procedure will not fully evaluate the relationship of all errors to one another. For example, if there is an error at the end of one increment of tape, there is no basis for relating that error to another error immediately following that increment but falling within the next increment of tape. As indicated hereinafter, the inventor now believes a faulty aspect of such test procedures known to the art to be their failure to evaluate the full potential for harm that each error possesses.